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The state of the art of current hand-held tire air pressure gauges does not provide a useful measure of the proper tire inflation pressure if the vehicle has been recently driven. Driving causes flexing/friction within the tire and that results in elevated tire internal temperatures and related higher tire pressures. Such increases can be significant: Tire pressures can increase by 2-3 psi in only a couple of miles of local driving and, after prolonged highway driving at high speeds, can result in increases of six psi or more. In fact, typical vehicle and tire manufactures"" instructions indicate that tire pressures should only be measured after the vehicle has been standing for at least three hours. The problem, however, is that tire inflation facilities are generally located remotely from the vehicle owner and the tire temperatures and pressures have become inflated from driving the vehicle to the gas station or on a trip. If the tire pressures have increased by five pounds due to highway driving, for example, a current tire gauge would indicate that the tires are over-inflated and may lead the user to release air from the tires and cause them to be under-inflated by five pounds.
Improperly inflated tires on a vehicle can cause increased tire wear, decreased fuel mileage, poor handling, unsafe driving conditions, and even the possibility of a tire blow-out. These concerns led to this hand-held digital tire pressure gauge that is designed to correct for any elevated tire temperatures/pressures that are due to driving the vehicle.
A hand-held digital tire air pressure gauge has a spring-loaded piston that is compressed by air pressure from the tire to allow a calibrated volume of inner air from the tire to enter the gauge where sensors measure the tire gauge pressure and the temperature of the sampled air from the tire. A separate sensor mounted externally on the gauge body measures the ambient (outside of the tire) air temperature and the difference between the measurements of the two temperature sensors is determined. This temperature difference, along with the tire air gauge pressure and an adjustment factor derived from the known behavioral relationship between air pressure and air temperature, are analyzed by a series of digital electronic calculations in an equation to calculate a xe2x80x9ccorrectedxe2x80x9d tire air pressure which is indicated on a digital display. (See the specifics of this equation in the detailed description for FIG. 1E and also in FIG. 2) This corrected pressure is the air pressure that the tire would have if it had the same internal temperature as the temperature of the air outside of the tire.
The equation described in this application assumes that there is a linear relationship between air pressure change and air temperature change, and this assumption is probably sufficiently accurate in the temperature ranges expected for this application. However, nonlinear relationships could also be accommodated.
As previously pointed out in the background section, driving a vehicle will result in an increase of the air temperatures within the tires. This gauge, however, by correcting for this type of elevated tire temperatures, will allow the vehicle owner to drive to a service station or to check the tire pressures while on a trip and to still obtain valid tire air pressure readings. For example, if the vehicle manufacturer recommended thirty pounds of pressure when the tire is xe2x80x9ccoldxe2x80x9d, and this gauge indicated a xe2x80x9ccorrectedxe2x80x9d tire pressure of twenty-five pounds when the tire is hot due to highway driving, the user could confidently add five pounds of air to the tire and know that the tire will be properly inflated. If he adds the five recommended pounds and later checks the tire when the vehicle has not been recently driven, the gauge will indicate the correct tire pressure of thirty pounds.
The design of the piston/spring/displacement chamber mechanism within the gauge ensures that a sufficient volume of tire air enters the displacement chamber for an accurate tire air temperature measurement without allowing a significant loss of tire air pressure (estimated loss of less than one-eighth of a psi for the specifications used here). No special action is required by the user other than pressing the gauge against the tire stem as he would with any hand held digital tire air pressure gauge. Further, since the piston movement is driven first by tire air pressure and then by a return spring, there is no battery power required and thus no drain on the gauge battery during its operation. The external temperature sensor is located at the opposite end of the gauge from the tire temperature sensor to ensure that there is no temperature influence from the tire. The external temperature sensor is also protected by a plastic grid that is designed such that it discourages nearby hand placement that could unduly influence the sensor reading.